1. Field of the Invention
The present invention relates to a method of fabricating a multipole lens having multipole elements, such as octopoles or dodecapoles and also to the multipole lens. Furthermore, the present invention relates to a charged-particle beam instrument fitted with such multipole lenses.
2. Description of Related Art
In a charged-particle beam instrument such as a scanning electron microscope, aberration in the electron beam that is a charged-particle beam is corrected when the beam is directed at a specimen, in order to appropriately image the specimen. A multipole lens is mounted as an aberration corrector within this charged-particle beam instrument to correct the aberration in the beam.
A known construction of this multipole lens as shown in Japanese Patent Laid-Open No. H2-230647 has plural (e.g., 8 or 12) multipole elements which are supported by an annular holding member and a yoke disposed outside the holding member.
This multipole lens (multipole elements) described in the above-referenced Japanese patent application is described below. The multipole lens shown in FIG. 1 of the above-referenced Japanese patent application has mounting rods, plural polar elements consisting of polepieces coupled to the front ends of the mounting rods, a beam tube, and an annular yoke (annular iron circuit) disposed outside the beam tube. This tube is provided with holes to permit passage of the mounting rods of the polar elements and to form a hermetic seal.
Each polar element is formed by screwing the front-end portion of the mounting rod into the polepiece or adhesively bonding or welding them together. The mounting rods and polepieces forming the polar elements are made of a magnetic material. The beam tube is made of an electrically insulating material. A metallic coating is formed around the holes.
The mounting rods of the polar elements are firmly coupled to the yoke at their base ends. The mounting rods are hermetically and rigidly mounted within the holes in the beam tube by welding via the metallic film. Thus, this weldment forms a hermetically sealed body.
A coil is mounted on each mounting rod between the beam tube and yoke. The polepieces bonded to the front-end portions of the mounting rods are excited by electrically energizing the coils. The base-end portions of the mounting rods are inserted in the holes of the yoke via insulators. Electric terminals are connected with their respective base-end portions, and voltages are supplied.
During fabrication of the multipole lens of the construction described above, the mounting rods forming the polar elements and the holes in the beam tube are firmly bonded together by welding. At this time, the polar elements have been placed in position. At least the welded portions are heated. The heat applied during this welding is transmitted also to the beam tube by thermal conduction and to the mounting rods forming the polar elements. Furthermore, the heat is transmitted to the polepieces at the front ends of the rods.
For this reason, during the welding, the polar elements and beam tube expand thermally. If the heat is subsequently dissipated away and they return to room temperature, their shapes might not be completely returned to their original shapes. In this case, the polar elements are deformed from the state in which they have been previously placed in position. Therefore, after the welding step, it is necessary to modify the shapes of the polar elements by cutting. In this way, labor is required. Hence, the manufacturing efficiency is not good.